rustc now generates the coverage map and can support (limited)
coverage report generation, at the function level.

Example:

$ BUILD=$HOME/rust/build/x86_64-unknown-linux-gnu
$ $BUILD/stage1/bin/rustc -Zinstrument-coverage \
$HOME/rust/src/test/run-make-fulldeps/instrument-coverage/main.rs
$ LLVM_PROFILE_FILE="main.profraw" ./main
called
$ $BUILD/llvm/bin/llvm-profdata merge -sparse main.profraw -o main.profdata
$ $BUILD/llvm/bin/llvm-cov show --instr-profile=main.profdata main
    1|      1|pub fn will_be_called() {
    2|      1|    println!("called");
    3|      1|}
    4|       |
    5|      0|pub fn will_not_be_called() {
    6|      0|    println!("should not have been called");
    7|      0|}
    8|       |
    9|      1|fn main() {
   10|      1|    let less = 1;
   11|      1|    let more = 100;
   12|      1|
   13|      1|    if less < more {
   14|      1|        will_be_called();
   15|      1|    } else {
   16|      1|        will_not_be_called();
   17|      1|    }
   18|      1|}

`rustc` allows passing in predefined target triples as well as JSON
target specification files. This change allows bootstrap to have the
first inkling about those differences. This allows building a
cross-compiler for an out-of-tree architecture (even though that
compiler won't work for other reasons).

Even if no one ever uses this functionality, I think the newtype
around the `Interned<String>` improves the readability of the code.

Since #73374 the rustc wrapper no longer configures debug assertions
based on RUSTC_DEBUG_ASSERTIONS environment variable.

This should run much faster.

There are also some drive-by cleanups here to try to simplify things.
Also, the paths for in-tree crates are now displayed as relative
in `x.py test -h -v`.

Clippy's tests were failing the build, but that failure was ignored in favor of
checking toolstate. This is the correct behavior for toolstate-checked tools,
but Clippy no longer updates its toolstate status as it should always build.

Since cargo appends executable/args, the support_lib count
parameter has to come first.

This just unwraps clippy's build step instead of skipping tests if clippy didn't
build. This matches e.g. cargo's behavior and seems more correct, as we always
expect clippy to successfully build.

it's not been built since a long time ago

So that the rlibs will work with both LTO and non-LTO builds.

This is a follow-up to #70123, which added `llvm-config --libdir` to the
`LIBRARY_PATH` for rustc tools. We need the same for "run-make-fulldeps"
and "ui-fulldeps" tests which depend on compiler libraries, implicitly
needing to link to `-lLLVM` as well.

Without `Compiletest.path` set running `x.py test src/test/rustdoc-js` would run the `rustdoc-js` test suite with everything filtered out.

As this was the only place setting `Compiletest.path` to `None` this removes the `Option` wrapper as well.